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Gamma-ray astronomy / Astronomie des rayons gamma – Vo
The future of gamma-ray astronomy
L’avenir de l’astronomie gamma
Jürgen Knödlseder
IRAP, 9, avenue du Colonel-Roche, 31028 Toulouse cedex 4, France
Comptes Rendus Physique, Vol. 17, Issue 6, pp. 663-678
a r t i c l e
Article history:
Available online xxxx
Keywords:
i n f o
a b s t r a c t
The field of gamma-ray astronom
decade. Thanks to the advent of
(H.E.S.S., MAGIC, VERITAS) and th
thousand gamma-ray sources ar
Observing gamma rays
photoelectric effect
Compton scattering
pair creation
lenses
pair convertors Cherenkov telescopes
Compton telescopes
coded masks
Particle detectors
space-based
TeVPA2016(12-16September2016)
ground-based
2
History of gamma-ray astronomy
Number of detected sources in red
177
>3000
2010
HAWC
Fermi
INTEGRAL
32
2000
H.E.S.S.
271
COMPTEL
1980
MILAGRO
B. Sripathi Acharya
Early History
The TIFR group started the field of ground-based -ray astronomy in India in 1969, soon after the announcement of the discovery of pulsars in 1968. Pulsars were proposed to be the site of origin of high energy cosmic
rays and hence the sources of ultra high energy -rays. They searched for pulsed emission of -rays from 5
pulsars with the same periodicities as observed at radio frequencies [1].
25
CGRO
COS-B
1970
VERITAS
The Cherenkov light was detected by 2 large search light mirrors ( f/0.45 mirrors of 90 cm dia) focused onto
fast (56 AVP) photo-multipliers (PMT) placed at their foci. The full field of view of each mirror was . The
approximate collection area of high energy -rays was
and the threshold energy of detection
. The mirrors, mounted on an orienting platform, tracked each pulsar for about an hour when it
is close to meridian transit. Observations were made on clear nights in the years 1969 and 1970 at Mukurthi
located at an altitude of 2.2 km above mean sea level in the Nilgiri Hills in South India. Not having found
any significant flux of -rays from any source they placed upper limits on the flux of -rays of the order of
[2].
Whipple
9
HEGRA
….............
SMM
272
2.
EGRET
1990
CELESTE
MAGIC
This activity was revived after a hiatus of 5 years with more number of mirrors and improved detection methods.
For 10 years, an array of Cherenkov telescopes with a total mirror area of about
was operated in
and around Ooty (Now called Udhagamandalam) in southern India (
,
, 2.3 km altitude).
A photograph of the set-up is shown in figure 1. Later, this set-up was moved in the year 1986 to Pachmarhi
(
,
, 1075 m altitude) in the state of Madhya Pradesh, in central India, where the sky conditions
for night sky observations are better than that at Ooty. A very systematic search for pulsed emission of -rays
was made on a number of isolated pulsars [3] and X-ray binaries [4] using these set-ups at Ooty and.Pachmarhi.
Crab, Vela, PSR0355+54, Geminga and Her X-1 showed positive signals on several occasions [5, 7, 6, 8, 9,
10, 11].
Thémistocle
2
HEAO 3
SAS 2
Ooty
Figure 1. Compact array at Ooty
1960
balloons
1
Ranger 3
OSO 3
Explorer XI
The BARC group started observing the night sky in the mid seventies at Gulmarg in Kashmir (
,
,
2743 m altitude) using two bare faced PMTs (EMI 9545B) separated by 1 m and pointing up-wards in the sky
(which may be likened to a wide angle telescope with a viewing angle of
w.r.t. zenith) in the drift scan
mode. In this mode the telescope is kept stationary, and the sky is scanned as Earth rotates. They reported [12] a
non-random component in the arrival times of cosmic ray events for time separation of
. Later following
the TIFR group, they too installed [13] a similar set-up at Gulmarg, consisting of 6 equatorially mounted
TeVPA2016(12-16September2016)
Crimean
AERE
Harwell garbage bin Observatory
3
Nucleosynthesis in
the Universe
2010
2000
2014:
56Co
Achievements
lines from SN Ia
2003: 60Fe lines from Galaxy
1994: 44Ti lines from Cas A
1990
1960
2013: Proton acceleration in SNR
2011: Crab nebula flares
2010: Fermi bubbles,
First radio galaxy lobe, First nova
2009: First millisecond pulsars,!
First starburst galaxies
1993: Galactic origin of cosmic rays
1992: Diffuse LMC emission
1973: GRB, solar deexcitation lines
1972: e+e- 511 keV line
Cosmic accelerators
2009: First starburst galaxies
2008: Crab pulsar
2005: First binary
2004: First resolved SNR
2002: First unidentified TeV source
2000: First SNR
1992: Mkn 421
1989: Crab nebula
1988: 56Co lines from SN II
1984: 26Al line from Galaxy
1980
1970
Cosmic rays in the
Galaxy and beyond
1981: 25 point-like sources
1978: First blazar
1974: Crab pulsar
1972: Diffuse Galactic emission
1969: Crab pulsar
1962: Cosmic background
1961: 22 photons > 50 MeV
1958: Solar flare
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4
Scientific Challenges
The nature of Dark Matter
• 
• 
• 
Indicates a major flaw in our
understanding of nature
Proposed solutions include
new fundamental particles
(WIMPs, axions, etc.)
Decay products of these
particles may be detectable
in gamma rays
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5
Scientific Challenges
The nature of Dark Matter
• 
• 
• 
Indicates a major flaw in our
understanding of nature
Proposed solutions include
new fundamental particles
(WIMPs, axions, etc.)
Decay products of these
particles may be detectable
in gamma rays
The origin of Cosmic Rays
• 
• 
• 
• 
Unveiling the Galactic
PeVatrons
Impact of low-energy cosmic
rays on interstellar chemistry
Cosmic-ray propagation
Impact of environment
TeVPA2016(12-16September2016)
6
Scientific Challenges
The nature of Dark Matter
The origin of Cosmic Rays
The physics of Particle
Acceleration
• 
• 
• 
• 
Indicates a major flaw in our
understanding of nature
Proposed solutions include
new fundamental particles
(WIMPs, axions, etc.)
Decay products of these
particles may be detectable
in gamma rays
• 
• 
• 
• 
• 
• 
What mechanisms are
actually at operation in a
given source?
Insights from variability !
(time domain astronomy)
Elusive source classes
Unveiling the Galactic
PeVatrons
Impact of low-energy cosmic
rays on interstellar chemistry
Cosmic-ray propagation
Impact of environment
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J. Knödlseder / C. R. Physique ••• (••••) •••–•••
5
Space-based projects
Table 1
Summary of instruments and mission concepts for space-based gamma-ray astronomy (see text). MDP indicates the minimum detectable polarization of an
instrument. Detector technologies comprise time projection chambers (TPC), silicon trackers (Si), cesium iodide scintillators (CsI), scintillating fibers (fib.),
lead tungstate scintillators (PbWO4 ), bismuth germanate scintillators (BGO), lutetium yttrium orthosilicate scintillators (LYSO), and magnetic spectrometers (B).
Parameter
AdEPT
e-ASTROGAM
Context
Launch date
Energy range (GeV)
Ref. energy (GeV)
!E/E
A eff (cm2 )
Sensitivity (mCrab)
Field of view (sr)
Angular resolution
MDP (10 mCrab)
Technology
R&D
–
0.005–0.2
0.07
30%
500
10
t.b.d.
1◦
10%
TPC
M5?
2029?
0.0003–3
0.1
30%
1500
10
2.5
1.5◦
20%
Si + CsI
CALET
ISS
2015
launched
0.02–10000
100
2%
t.b.d.
1000
1.8
0.1◦
–
fib. + PbWO4
DAMPE
GAMMA-400
HARPO
HERD
PANGU
China
Russia
∼2021
0.1–3000
100
1%
5000
100
1.2
0.02◦
–
Si + CsI
R&D
–
0.003–3
0.1
10%
2700
1
t.b.d.
0.4◦
t.b.d.
TPC
China
>2020
0.1–10000
100
1%
t.b.d.
10
t.b.d.
0.1◦
–
Si + LYSO
ESA/CAS?
2021?
0.01–5
1
30%
180
t.b.d.
2.2
0.2◦
t.b.d.
Si (fib.) + B
2015
launched
2–10000
100
1.5%
3000
100
2.8
0.1◦
–
Si + BGO
•  Detection sensitivities are still poor in the MeV domain
•  Considerable
potential
exists
in using
modern,
pixelised
semiconductor
production
interactions.
This would
lead
to a system
that space-proven
is sensitive fromhighly
the MeV
to the GeV
domain, going about more
a compact
configuration
with
a minimum
of passive
material
detect gamma
than detectors
a factor 10in
beyond
predecessor
instruments,
opening
up thisamount
poorly explored
energy
band. to
In addition,
such a system
rays
through to
Compton
and pair
interactions
would
be sensitive
the polarization
of creation
incoming gamma
rays, accessing a new physical dimension that provides invaluable
information about the underlying emission processes and source geometries.
energies, succeeding
Fermi-LAT
willwill
be be
challenging.
The (Fermi
Fermi-LAT
tracker has
a geometric
area of 1.5 ×
•  AtAthigher
GeV energies,
succeedingtoto
Fermi-LAT
challenging
spacecraft
weight
is 4.3 tons,
2
1.5 m
, the spacecraft
a diameter
of detector)
2.5 m, a height of 2.8 m and a mass of 4.3 tons, which is a very respectable satellite.
difficult
to buildhas
a much
bigger
Making
the
much is
bigger
to increase
the (i.e
effective
area wouldcan
soon
the maximum
capacities of
•  Area
oftelescope
improvement
angular
resolution
pointdetection
spread function);
be hit
achieved
by decreasing
existing
(and also
planned)
launch
vehicles.spacing
So the detection
area
cannotand
be substantially
density
of tracker
and
increasing
between
tracker
calorimeterexpanded. Significant improvement
in sensitivity for pair-creation telescopes can only be achieved through a dramatic improvement in the angular resolution,
especially at lower energies where the Fermi-LAT point spread function (PSF) exceeds several degrees. The best ways to
•  Potential to cover both aspects in a single mission
improve the PSF are to decrease the density of the material in the tracker and to space the tracking element further
apart [62]. This joins pretty much the needs expressed above for the low- to medium-energy domains, and both needs can
be combined in a single mission providing a low-energy extension to Fermi-LAT.
The following sections will describe some of the proposed instruments and mission concepts for the future study of the
gamma-ray Universe from space. Table 1 provides
a summary of the performance of the proposed instruments and mission
TeVPA2016(12-16September2016)
8
concepts, as presented in the relevant references. The accuracy of the performance predictions varies between instruments
and also depends on the maturity of the project, hence the table is only useful for order-of-magnitude comparisons. The
Time Projection Chamber (TPC)
30cm)3 cubic TPC
drift
ee-
Time Projection Chambers
E
Up to 5 bar.
signal
amplification
and
collection
+ e-
e
-
27
e
AdEPT
Micromegas + GEM gas amplification
HARPO
ollection on x, y strips, pitch 1 mm.
FTER chip digitization, up to 50 MHz.
cintillator / WLS / PMT based trigger
micromegas + (1 or 2) GEM
5
3
10
102
Few MeV – GeV domain, polarimetry
Pair conversion telescope
Gas
(Ar-based)
filled TPC10(few bars
pressure)
V
= 250V,
E = 250V/cm
V
E
V
sect MCA
E
V
= 250V, E = 250V/cm
400 250 270 50
5
5V
3
Target volume: 200 x 200 x 100 cm
V
= 250V, E = 250V/cm
V
= 250V, E = 750V/cm
e-e+ pairs drift downwards to detector
plane
10
µ ∝ A , A=2.08±0.05
Detectors
•  Gas electron multiplier and micro-well
detector (AdEPT)
200 •  400Micro-mesh
600
800
1000
340
350
360detector
370
380
390
400
410
and microstrip
V
[V]
charge [ADC]
(HARPO)
Prototypes built and tested
Stratospheric balloon flights planned
AdEPT proposed as MIDEX in U.S.
COST 2014
12
mesh
b
trans
b
GEM
drift
5
4
gem
t
gem
t
gem
t
gem
t
VGEM
20V
Main peak, µM only
Fe & cosmics caracterization
10
Main peak, µM+1GEM
Escape peak, µM+1GEM
1
0
h. Gros, TIPP2014
rnard
10
4
• 
• 
• 
• 
• 
• 
total gain
events/second
IM A 695 (2012) 71, NIM A 718 (2013) 395
mesh
Figure 2 - Schematic of the 3-DTI TPC technology showing how electron-positron pairs are
tracked in 3-D. Components of the time projection chamber, micro-well detector plus
GEM, and an avalanche in a single well are identitifed.
HARPO
• 
• 
• 
TeVPA2016(12-16September2016)
9
Time Projection Chambers
Angular resolution improvement
TeVPA2016(12-16September2016)
Polarisation
10
e-ASTROGAM
Sub MeV – GeV domain, polarimetry
Compton and pair conversion telescope
Detector
•  Double sided Silicon strip (DSSD) tracker
•  3D imaging scintillator CsI(Tl) calorimeter !
read out by Si drift diodes
•  Plastic anticoincidence shield read out by
SiPM
Using technology heritage from existing satellites
Proposed as ESA M5 mission
Similar concept proposed as NASA MIDEX (ComPair)
• 
• 
• 
e-ASTROGAM Measurement"
• 
• 
• 
Pair event
γ
Silicon
tracker
Compton event
γ
Scintillator
calorimeter
θ
Plastic anticoincidence system
Track
e+
e-
TeVPA2016(12-16September2016)
11
Calor
e-ASTROGAM hardware
Payload"
e-ASTROGAM
Detail"of"the"detector-ASIC"
bonding"in"the"AGILE"Si"Tracker"
8"
•  Tracker:"56"layers"of"4";mes"5×5"DSSDs"(5 600"
in"total)"of"500"µm"thickness"and"240"µm"pitch"
•  DSSDs"bonded"strip"to"strip"to"form"5×5"ladders"
•  Light"and"s;ff"mechanical"structure"
•  Ultra"low-noise"front"end"electronics"
"
Sandwich
panel
PICSiT!CsI(Tl)!pixel!Design of the calorimeter composed of
1 module of
•  Calorimeter:"8 464"CsI(Tl)"bars"coupled"at"both"
14x14 modules of 64 crystals
64 crystals
ends"to"low-noise"Silicon"Dria"Detectors"
•  ACD:"segmented"plas;c"scin;llators"coupled"to"
SiPM"by"op;cal"fibers"
•  Heritage:"AGILE,"Fermi/LAT,"AMS-02,"INTEGRAL,"
Fermi/LAT"AC"system"
LHC/ALICE…"
"
V.!Ta:scheff!for!the!e%ASTROGAM!Collabora:on!!!
!!!!!!!!!!!!28th!Texas!Symposium! !
TeVPA2016(12-16September2016)
!
!13 18!December!2015!
12
•  Improve"angular"resolu;on"close"
to"the"Compton"physical"limits"
Simula;on" of" the" Cygnus" region"
in" the" 1" " 3" MeV" energy" band"
using"the"e-ASTROGAM"PSF,"from"
an" extrapola;on" of" the" 3FGL"
source"spectra"to"low"energies"
Angular resolution (degree)
potential
Angular"resolu;on"
e-ASTROGAM
e
-ASTROGAM science
6"
10
Fermi/LAT
COMPTEL
1
Compton
Pair
e-ASTROGAM
10
-1
10
-1
1
10
10
2
10
3
10
4
Gamma-ray energy (MeV)
V.!Ta:scheff!for!the!e%ASTROGAM!Collabora:on!!!
!!!!!!!!!!!!28th!Texas!Symposium! !
TeVPA2016(12-16September2016)
!
!13 18!December!2015!
13
Some other projects
Pangu
HERD
Gamma-400
• 
• 
• 
• 
• 
• 
Fermi/LAT-like with
increased spacing between
tracker and calorimeter
Better angular resolution
Poorer sensitivity
Status unclear
• 
• 
• 
• 
Primarily a particle detector
(like CALET, DAMPE)
GeV (- TeV) domain
3-D cubic calorimeter
surrounded by microstrip
silicon trackers from five sides
Weight limited to 2 tons (half of
Fermi-LAT)
Will be placed aboard Chinese
space station (2020+)
TeVPA2016(12-16September2016)
• 
• 
• 
Few MeV – GeV domain,
polarimetry
Pair conversion telescope
Tracker combined with
magnetic spectrometer to
fit into 60 kg payload
allocation
Submitted unsuccessfully
to joint ESA/CAS small
mission call
14
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Ground-based projects
J. Knödlseder / C. R. Physique ••• (••••) •••–•••
10
Table 2
Summary of future observatories and experiments for ground-based gamma-ray astronomy.
Parameter
CTA
Site(s)
Altitude (m)
Latitude
Start of operations
Lifetime (years)
Energy range (TeV)
!E/E
A eff (m2 )
Sensitivity (mCrab)
Field of view
Angular resolution
t.b.d.
∼ 2000
t.b.d.
2020
30
0.02–300
10%
3 × 106
1
5◦ –10◦
0.05◦
• 
• 
• 
• 
• 
• 
• 
HAWC
Sierra Negra (Mexico)
4100
19◦ N
started
2013
10
0.1–100
50%
30 000
50
1.8 sr
0.5◦
HiSCORE
LHAASO
MACE
Tunka Valley (Russia)
675
51.8◦ N
t.b.d.
t.b.d.
50–10 000
10%
108
100
0.6 sr
0.1◦
Daocheng (China)
4300
29◦ N
2020?
> 10
0.1–1000
20%
8 × 105 (KM2A) 106 (WCDA)
10
1.5 sr
0.3◦
Hanle (India)
4270
32.8◦ N
2016
t.b.d.
t.b.d.
t.b.d.
t.b.d.
t.b.d.
4◦
t.b.d.
Imaging Air Cherenkov Telescopes (IACTs) have been proven most efficient to study gamma-ray
induced atmospheric Cherenkov light (excellent angular resolution, strong background rejection
power)
Drawbacks are low duty cycles (~10%) and narrow fields of view (~5°)
Performance increase through more telescopes covering a larger area and eventually using SiPM
instead of PMTs
Water Cherenkov Detectors (WCDs) are most successful devices for studying the tails of extended air
showers (“tail catcher detectors”)
While modest in angular resolution and background rejection, they have excellent duty cycles and
wide field of view (complementary to IACTs)
Performance increase through larger surface areas, moving the detector to higher altitude, and
improving the detector configuration
Open access observatories
Fig. 3. Artists view of a CTA array site.
TeVPA2016(12-16September2016)
15
Cherenkov Telescope Array
Figure 2.3 – Artist impression of the southern CTA site, showing the array combining three different sizes of Cherenkov telescope, covering a multi-km2 area. The combination of a large number of telescopes of different sizes gives CTA tremendous
flexibility of operation as well as unprecedented sensitivity and energy range.
SST
(!4m)
questions. These three telescope sizes are referred to as the Large-Sized, Medium-Sized and SmallSized Telescopes (LSTs, MSTs and SSTs). The basic characteristics of these telescopes were fixed as
requirements based on these early simulations, plus a first very large scale simulation of realistic CTA
telescope arrays. Later large scale simulations served to assess the site-dependence of performance
(in particular site altitude and geomagnetic field) and to establish appropriate telescope spacing. A final
very large (200M HS06 CPU hours, 3 PB disk space) effort was used to perform a final fine-tuning of
MST
the layouts, which are presented in Figure 2.4. The chosen layouts may need to be adapted for ease of
(!12m)
deployment,
analysis
LST in particular for La Palma, which has more complex constraints. All simulation and
tools used have been verified on operating Cherenkov telescope arrays and multiple independent chains
(!23m)
have been compared at all steps in the process.
Paranal/Chile
Type:
La Palma
La Palma/Spain
(3AL4M15-5)
Type:
23-m LST
23-m LST
12-m MST
12-m MST
4-m SST
(MAGIC)
North (x) is up
West (y) is left
North (x) is up
West (y) is left
250 m
1000 m
Circle:
- 400 m
Circles:
- 400 m
- 800 m
- 1200 m
4 LSTs, 25 MSTs, 70 SSTs
TeVPA2016(12-16September2016)
4 LSTs, 15 MSTs
Figure 2.4 – Agreed layout of telescopes on the two sites in the southern hemisphere (left) and northern hemisphere (right).
The southern site contains more telescopes and covers a larger area. The SST array has a large footprint to provide a very
large gamma-ray collection, but with a relatively high energy threshold due to the smaller telescope size. The LSTs have
16
Cherenkov Telescope Array
An Open Observatory
2. The CTA Observatory
A world-wide endeavour
Improvements everywhere
Figure 2.1 –TeVPA2016(12-16September2016)
Illustration of performance goals and associated science drivers.
2.2 Telescope Arrays
17
Large size telescopes
Science drivers
• 
• 
• 
Lowest energies (< 200 GeV)
Transient phenomena
DM, AGN, GRB, pulsars
Characteristics
• 
• 
• 
• 
• 
• 
• 
• 
• 
Parabolic design
23 m diameter
370 m2 effective mirror area
28 m focal length
1.5 m mirror facets
4.5° field of view
0.11° PMT pixels
active mirror control
Carbon-fibre arch structure (fast repointing)
Array layout
• 
• 
South site: 4
North site: 4
Status
• 
• 
Some elements prototyped
First full telescope under construction in !
La Palma (http://www.lst1.iac.es/webcams.html)
TeVPA2016(12-16September2016)
18
Mid size telescopes
Science drivers
• 
• 
Mid energies (100 GeV – 10 TeV)
DM, AGN, SNR, PWN, binaries,!
starbursts, EBL, IGM
Characteristics
• 
• 
• 
• 
• 
• 
• 
Modified Davies-Cotton design
12 m diameter
90 m2 effective mirror area
1.2 m mirror facets
16 m focal length
8° field of view
0.18° PMT pixels
Array layout
• 
• 
South site: 25
North site: 15
Status
• 
• 
Telescope prototyped (Berlin-Adlershof)
Prototype cameras under construction (2 types: NectarCAM & FlashCam)
TeVPA2016(12-16September2016)
19
Small size telescopes
ASTRI
SST 1M
GCT
Science drivers
• 
• 
Highest energies
(> 5 TeV)
Galactic science,
PeVatrons
Array layout
• 
• 
South site: 70
North site: -
First CTA light
Characteristics
• 
• 
• 
• 
• 
• 
Davies-Cotton design
4 m diameter
8.5 m2 effective mirror area
5.6 m focal length
9° field of view
0.24° SiPM pixels
Status
• 
• 
Prototype telescope built
Camera prototype under
construction
Characteristics
Characteristics
Status
Status
• 
• 
• 
• 
• 
• 
• 
• 
• 
Schwarzschild-Couder design
4.3 m primary diameter
1.8 m secondary diameter
6 m2 effective mirror area
2.2 m focal length
9.6° field of view
0.17° SiPM pixels
• 
• 
• 
• 
• 
• 
• 
Schwarzschild-Couder design
4 m primary diameter
2 m secondary diameter
6 m2 effective mirror area
2.3 m focal length
8.6° field of view
0.16° SiPM pixels
•  Prototype telescope structure built
Prototype telescope built
•  Tested with MAPMT-based CHEC camera
Camera prototype under
construction
TeVPA2016(12-16September2016)
20
LHAASO – A multi component expe
Some other projects
HiSCORE
LHAASO
MACE
Four 90000 m2
Water Cherenkov
detectors. Each
one has the size
of HAWC
• 
• 
• 
• 
Non-imaging air-shower
Cherenkov light-front
sampling
Up to 100 km2 area covered
Wide field of view (~0.6 sr)
Extend sensitivity to the
PeV regime
Complemented by IACTs
and surface & underground
stations for measuring
muon component of air
showers
24 Wide FOV air
Cherenkov image
• 
Telescopes
400 burst detectors
for high energy
• 
• 
Hybrid detector array
Gamma ray detectors
•  Large (4 x HAWC) Water
Cherenkov detector array
(0.1-30 TeV)
•  Electromagnetic particle
detectors and muon
detectors (30-1000 TeV)
TeVPA2016(12-16September2016)
• 
• 
21 m diameter IACT to be
installed at Hanle (4270 m a.s.l)
Design inspired from H.E.S.S. II
LHAASO
2
1 Km array
4300 m
21
Sensitivity: past – present – future
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[m3G; v1.175; Prn:5/05/2016; 11:50] P.14 (1-16)
J. Knödlseder / C. R. Physique ••• (••••) •••–•••
Fig. 5. Differential 5σ sensitivity of gamma-ray telescopes (1 erg cm−2 s−1 = 1 mW m−2 ). Future instruments are shown as solid lines, concepts based on
TeVPA2016(12-16September2016)
R&D work are shown as dashed-dotted lines and existing or
past instruments are shown as dashed lines. Colors distinguish the energy22domains (blue for
low and medium energy, green for high energy, red for very high energy and magenta for ultra high energy. The grey lines show the Crab differential energy
flux, as well as 10%, 1% and 0.1% of that flux for reference. The lowest of the lines corresponds to a differential energy flux of 1 mCrab. The INTEGRAL
JID:COMREN AID:3304 /SSU
Conclusions
[m3G; v1.175; Prn:5/05/2016; 11:50] P.14 (1-16)
J. Knödlseder / C. R. Physique ••• (••••) •••–•••
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Space-based
• 
• 
Ground-based
An instrument covering the MeV – GeV energy
range has the highest discovery potential!
(e.g. e-ASTROGAM, ComPair)
Will enable
•  measurement of pion-bumps characteristic of
hadronic accelerators in many sources
•  study of the still elusive low-energy cosmic-ray
component
•  observation of gamma-ray lines
(nucleosynthesis, de-excitation, !
e+e- annihilation)
•  gamma-ray polarisation measurements
Fig. 5. Differential 5σ sensitivity of gamma-ray telescopes (1 erg cm−2 s−1 = 1 mW m−2 ). Future instruments are shown as solid lines, concepts based on
R&D work are shown as dashed-dotted lines and existing or past instruments are shown as dashed lines. Colors distinguish the energy domains (blue for
low and medium energy, green for high energy, red for very high energy and magenta for ultra high energy. The grey lines show the Crab differential energy
flux, as well as 10%, 1% and 0.1% of that flux for reference. The lowest of the lines corresponds to a differential energy flux of 1 mCrab. The INTEGRAL
sensitivity is for a typical observing time of 106 s [100]. The COMPTEL sensitivity is for the observing time accumulated during the whole duration of the
CGRO mission (∼ 9 years). The AdEPT sensitivity is for a typical observing time of 106 s [63]. The e-ASTROGAM [66], HARPO [101], and Fermi-LAT [102]
sensitivities are given after 3 years of scanning observations for a source at high Galactic latitudes [66]. The GAMMA-400 sensitivity is for a 30 days long
observation of a source at high Galactic latitude [103]. The CALET, DAMPE and HERD sensitivities are for 1 year of observation time (adapted from [80]).
The MAGIC stereo system [7], H.E.S.S., and CTA [104] sensitivities are for an effective exposure time of 50 hours. The ARGO-YBL sensitivity is after 6 years
of observations [105]. The MILAGRO sensitivity is for 1 year of observations [90]. The HAWC sensitivity is after 5 years of observations [90]. The HiSCORE
sensitivity is for a 100 km2 array after 5 years of survey observations, equivalent to an on-source exposure time of 1000 hours [92]. The LHAASO sensitivity
is after 1 year of observations [106].
The Cherenkov Telescope Array will expand on all
aspects of current IACTs (sensitivity, energy range,
angular resolution)
•  Will enable
•  WIMP detections from few 100 GeV to few TeV
and physics of the still elusive low-energy cosmic-ray component. e-ASTROGAM furthermore covers the MeV domain that is
•  linessearch
of PeVatrons
the nucleosynthesis
entire Galaxy
rich of gamma-ray
which give access
to nuclear cosmic-ray in
excitations,
processes, and the positron–
electron annihilation features below ∼ 511 keV. The AdEPT and HARPO concepts are also promising in the medium- to
•  though
measurement
of gaseous
sub-minute
in toAGN
high-energy domains,
both employ large volume
time projection variability
chambers that still need
demonstrate
their compliance with a space environment. Stratospheric balloon flights, as planned for example for the AdEPT instrument,
•  comprehensive population studies of particle
are here crucial to test the reliability of the instrument in a space-representative environment.
Another important leap in sensitivity will be achieved in the very high energy domain by CTA, with a factor of ∼ 10
accelerators
increase with respect to current telescopes. One of the key features of CTA will be the extended energy range from a few
tens of GeV to above 100 TeV, which is nicely visible in Fig. 5. With a few 100 hours exposure time, CTA will reach milli•  studies of particle acceleration in and particle
Crab differential sensitivity, making it the most sensitive gamma-ray telescope ever. CTA will reach the expected gamma-ray
emission from annihilation
of WIMPs with massesnear
in the few
100 GeV to few TeVsources
domain, provided that their annihilation
propagation
individual
cross section corresponds to the thermal relic density value. CTA will reveal thousands of very high energy gamma-ray
• 
sources, enabling comprehensive population studies of particle accelerators in the Universe. For example, CTA will detect
gamma-ray emission from young supernova remnants located at the opposite edge of the Galaxy, accessing thus the entire Galactic volume to search for the still elusive Galactic PeVatrons. For steady sources, CTA will surpass the Fermi-LAT
sensitivity above ∼ 50 GeV. For short-time phenomena, such as gamma-ray bursts or gamma-ray flares, CTA will be several
orders of magnitude more sensitive than Fermi-LAT even at lower energies, owing to its huge effective detection area of
> 106 m2 (compared to ∼ 1 m2 for Fermi-LAT). CTA will thus open a new window for time-domain astronomy, probing for
example sub-minute variability in active galactic nuclei.
LHAASO will explore even higher energies than CTA, thanks to the KM2A detector that measures the muon content in
TeVPA2016(12-16September2016)
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